1) Field of the Invention
The present invention generally relates to an electrophotographic printer including a toner image carrying body such as a photosensitive drum, a dielectric drum or the like to which a toner image obtained from a toner development of an electrostatic latent image is electrostatically adhered and held, and in particular, relates to a toner image transferring device incorporated therein for electrostatically transferring the toner image from the toner image carrying body to a sheet or paper.
2) Description of the Related Art
As is well known, the electrophotographic printer typically carries out the processes of: producing a uniform distribution of electrical charges on a surface of a rotary photosensitive drum; forming an electrostatic latent image on the electrically charged surface of the photosensitive drum by optically writing an image thereon with a laser beam scanner, an LED (light emitting diode) array, an LCS (liquid crystal shutter) array or the like; visually developing the electrostatic latent image with a developer, i.e. toner, which is electrically charged to be electrostatically adhered to the electrostatic latent image zone; electrostatically transferring the developed visible image or toner image from the drum to a sheet or paper; and fixing the transferred image on the sheet or paper.
In the transferring process, the transfer of the toner image to the paper is carried out by using a transfer charger to feed an electric charge to the paper.
Namely, the transfer charger is disposed in the vicinity of the photosensitive drum, the paper is introduced into a clearance therebetween, and the transfer charger gives the paper an electric charge having a polarity opposite to that of the electric charge of the toner image, whereby the toner image is electrostatically transferred from the drum to the sheet or paper. Note, the transfer charger may be a corona discharger.
To increase the efficiency of the transfer of the toner image, the paper must be given an amount of electric charge on the order of about 320 .mu. C/m.sup.2, and thus the transfer charger is provided with a capacity needed to give the paper such an amount of electric charge. Nevertheless, in practice, leading and trailing edge portions of the paper have a large amount of electric charge of much more than 320 .mu. C/m.sup.2 because, when the paper is not intervened between the transfer charger and the photosensitive drum, the charge is given to the drum as a body to be charged. In particular, the photosensitive drum has an electric capacity of about 5 .mu. F/m.sup.2, which is larger than that (about 0.5 .mu. F/m.sup.2) of the paper, so that an amount of electric charge fed from the transfer charger to the drum is relatively large when the paper is not intervened therebetween. As a result, when the leading edge of the paper is introduced into the clearance between the transfer charger and the drum, or when the trailing edge thereof leaves therefrom, a portion of the electric charge to be fed to the drum is added to the leading or trailing edge portion of the paper.
When the leading edge portion of the paper is given a large amount of electric charge, it will be strongly adhered to the photosensitive drum due to an electrostatic attraction acting therebetween, and thus the paper is often entangled with the drum. Therefore, conventionally, for example, a pair of nail members are engaged with a surface of the photosensitive drum at the side edges thereof, to separate the leading edge of the paper therefrom and prevent the entanglement of the paper with the drum. Nevertheless, the entanglement of the paper may frequently occur because the pair of nail members are merely applied to the side edges of the paper, and thus the separation of the paper from the photosensitive drum is not effectively carried out in practice. When the nail members are engaged with the photosensitive drum surface at the middle position thereof, to carry out an effective separation of the paper from the drum, they are applied to a middle area of the paper on which the toner image has been transferred, and thus the transferred toner image can be damaged by the nail members.
It is also known that an AC charger eliminator, which may be a corona discharger, is disposed adjacent to the transfer charger, to partially eliminate the electric charge from the paper to which the toner image has been transferred, whereby the electrostatic attraction between the paper and the photosensitive drum can be weakened. Nevertheless, the leading edge of the paper cannot be effectively separated from the photosensitive drum due to the excessive charge thereof. Of course, an AC charge eliminator may be arranged so that the electric charge can be eliminated from the leading edge of the paper, but in this case, the toner image may be returned from the paper to the photosensitive drum.